Radiation which is used for cancer therapy is broadly divided into two groups, that is, a photon beam and a particle beam. A photon beam is a wave of light and is used as conventional radiation such as X-ray, gamma rays, etc. On the other hand, a particle beam is radiation which is made by utilizing a particle of an atomic nucleus of hydrogen or that of carbon, etc. Especially, in the medical field, a beam, which is obtained by accelerating ‘a proton’ which is obtained by ionizing a hydrogen atom or ‘a carbon ion’ (will be also referred as a heavy ion) which is obtained by ionizing a carbon atom, is mainly used.
A particle beam therapy is based on the following principle. When a cancer cell is irradiated by a particle beam, DNA of the cancer cell is damaged, and the cell which is damaged to the degree which is beyond its own repair ability gradually dies and eventually, will be discharged from the body. A photon beam type conventional radiation, such as X-ray, gamma rays, etc. acts along the surface of a body. On the other hand, a particle beam has the feature such that the absorbed dose reaches a peak just before when the particle beam stops. Therefore, by changing a level of energy, a position where the absorbed dose reaches a peak (will be referred as a Bragg peak) can be focused on cancer nidus. Consequently, only the cancer nidus can be intensively destroyed. As above mentioned, more excellent effect than that of conventional technology can be obtained. Consequently, a particle beam therapy is expected increasingly as a therapy which can maintain QOL (Quality of Life).
In order to effectively perform a particle beam therapy, it is necessary to form a beam while damage is not given to surrounding tissue and sufficient level of dose is given to affected tissue. An irradiation method for realizing the beam forming is divided broadly into a ‘broad beam irradiation method’ and a ‘scanning irradiation method’. The broad beam irradiation method is a method in which a beam which is enlarged once by using a scattering body or a ridge filter, etc. is formed by using a collimator or a bolus. On the other hand, the scanning irradiation method is a method in which a pencil beam which is a beam whose beam diameter is kept to be small is scanned so as to scan in a pointillist manner (spot scanning having ON/OFF of beam) or so as to scan in a one-stroke sketch manner (raster scanning: beam is ON in principal).
Here, regarding scanning irradiation, more details will be described, and a relationship with a treatment planning device will be described. As described in the above, a particle beam has the feature such that a particle beam has a Bragg peak having a depth which corresponds to a level of energy. In order to apply appropriate irradiation dose to tissue of an affected part having a depth in intra-corporeal direction, it is assumed such that the tissue of an affected part is virtually divided into a layer which corresponds to a level of beam energy, the above-mentioned spot scanning or raster scanning is performed for each layer. Regarding a Bragg peak of a particle beam, an amount of dose which is applied to a side which is deeper than the Bragg peak is approximately zero. However, to a side which is shallower than the peak, some dose is applied. Therefore, considering the effect, at each spot position of each layer, irradiation amount should be determined. A treatment planning device calculates or simulates for determining an irradiation amount of the each spot for the each layer in order to realize to apply appropriate dose based on information of three dimensional image of an affected part which was photographed in advance.
In performing actual scanning irradiation, there is another thing to be determined; sequence order of irradiating each spot at each layer, that is, how a scanning route is determined. In a raster scanning, when it is assumed such that irradiation is performed by following a track which is made by combining each spot, the above-mentioned can be applied. In general, a treatment planning device performs a function of determining the scanning route of scanning. Prior art regarding the scanning route of the scanning is disclosed in Patent Document 1 or Non-Patent Document 1.